30/07/2025 7:28 am
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I know that shift registers like the 74HC595 can be daisy-chained to expand outputs, but I’m wondering where the practical limit lies. Is the limit mainly due to propagation delay and timing issues, or do factors like power consumption, loading on the data and clock lines, and signal integrity also become major concerns as the chain gets longer?
Are there any general guidelines (such as maximum number of devices or total outputs) before performance or reliability starts to drop? I’d be interested to hear from anyone who has pushed the number of chained shift registers in a real project.
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31/07/2025 5:09 am
Daisy-chaining a large number of shift registers, such as the popular 74HC595, is technically possible, but there are practical limitations you need to consider. Each shift register introduces a propagation delay, and as the chain gets longer, these delays accumulate.
When chaining around 100 shift registers, the total propagation delay can become significant, requiring you to slow down the clock frequency considerably to ensure reliable data transfer. High-speed operation becomes nearly impossible at this scale without special measures.
Signal integrity is another major concern. Longer chains increase the length of the data and clock lines, which can result in voltage drops, reflections, and noise issues. To maintain clean signals, you will likely need to use buffers or repeaters at certain points in the chain, along with careful PCB layout and proper decoupling.
If your design truly requires controlling such a large number of outputs, consider whether a different approach might be more suitable. For example, I²C or SPI GPIO expanders with unique addressing can drastically reduce complexity.
Alternatively, you could use multiple smaller chains driven by separate microcontroller pins.
